Metals from Groups I and II and organometallic compounds thereof are commonly used to catalyze the polymerization of monomers into polymers. For example, lithium, barium, magnesium, sodium, and potassium are metals that are frequently utilized in such polymerizations. Organolithium compounds are widely used to initiate such polymerizations. Catalyst systems of this type are of commercial importance because they can be used to produce stereoregulated polymers. For instance, lithium catalysts can be utilized to catalyze the anionic polymerization of isoprene into cis-1,4-polyisoprene.
The polymers formed in such polymerizations are terminated with the metal used to catalyze the polymerization and are sometimes referred to as living polymers. They are referred to as living polymers because the polymer chains which are terminated with the metal catalyst continue to grow or live until all of the available monomer is exhausted. Polymers that are prepared by utilizing such metal catalysts have structures which are essentially linear. Such polymers have structures that do not contain appreciable amounts of branching. Rubbery polymers of this type have certain drawbacks in that their flow characteristics at room temperature are extremely high and in that their tensile strength and tear resistance in the unvulcanized state are very poor due to less chain entanglement among their molecular chains. Due to these characteristics, the processing of such rubbery polymers prior to vulcanization is sometimes difficult. In order to improve the cold flow characteristics, tensile strength, and tear resistance of such unvulcanized rubbers they are often crosslinked prior to processing and subsequent vulcanization. Such metal terminated rubbery polymers can, for example, be crosslinked by treatment with divinylbenzene or tin halides. The use of such chemical agents in effect endlinks the polymer chains.
It is also known in the art that metal terminated polymers can be endlinked by treating them with a stoichiometric amount of silicon tetrachloride. The endlinking of a lithium terminated polymer with silicon tetrachloride is illustrated in the following reaction scheme: ##STR1## wherein P represents polymer chains. As can be seen, one mole of silicon tetrachloride is required for every four moles of lithium terminated polymer chains. In other words, one mole of silicon tetrachloride is required for every four moles of lithium in the lithium terminated polymer being treated. This relationship must be stoichiometrically perfect in order to endlink every lithium terminated polymer chain in the polymer being treated.
The endlinking of metal terminated polymers improves numerous physical properties. However, conventional endlinked polymers typically have very high molecular weights which makes processing difficult. This is because the high molecular weight polymers formed which have long chain branches tend to be broken down during milling and/or mixing operations. This results in the polymer's molecular weight being reduced during such operations which apply shearing forces. The polymers utilized in such operations typically have higher molecular weights than the ultimate molecular weight desired in anticipation of the molecular weight loss which occurs during processing. Even though it is possible to compensate for molecular weight loss, the polymer degradation which occurs still has a very detrimental effect on the physical properties of the polymer.